1. Field of the Invention
The present invention relates to a pentagonal roof reflecting mirror assembly for use in a viewfinder optical system of a single lens reflex camera and more particularly to a dust free modular unit that is not only capable of preventing the entry of dust and debris into the inside hollow space thereof, but will be free from condensation.
2. Description of the Prior Art
As is generally known, a pentagonal roof type reflecting mirror assembly is made up of a pair of roof reflecting mirrors and a third reflecting mirror to transmit an image of the object scene to a viewer for aiming and focusing the camera. With the advent of more sophisticated 35 millimeter cameras, there has been an additional requirement to make the 35 millimeter camera as compact and lightweight as possible. In this regard, there has been suggestions to utilize a plastic housing structure for the pentagonal roof reflecting mirror assembly. As can be appreciated by a skilled photographer, the pentagonal roof reflecting mirror assembly is removable on more expensive cameras in order to permit an optional changing of the focusing screens to accomodate the particular desired type photography such as astro photography, dim light applications, dioptric and parallax focusing with close ups.
One suggestion to provide a lighter weight and economical plastic pentagonal roof reflecting mirror assembly can be seen in U.S. Pat. No. 3,603,200. Another example can be seen in U.S. Pat. No. 3,618,449 wherein side and rear wall members are provided to both reinforce the roof reflecting mirror and to prevent the introduction of undesirable or stray light. These plastic reflecting mirror assemblies are open at the bottom for coaction with the focusing screen and are subject to the entry of dust through the open bottom portion which will adhere to the mirror and effect the resulting image transmitted to a viewer. The prior art use of solid glass pentaprisms did not suffer this problem due to the internally reflecting facets of the glass prism. Additional prior art exists in the following: Japanese Utility Model publication 47-24420, published on Aug. 2, 1972; Japanese patent publication 47-4095, published on Feb. 4, 1972; Japanese Utility Model publication 47-20836, published on July 12, 1972; Japanese Utility Model publication 47-33315, published on Oct. 7, 1972; Japanese Utility Model publication 47-33317, published on Oct. 7, 1972; Japanese Utility Model publication 49-12602, published on Mar. 28, 1974; and East German Pat. No. 50,419, published on Apr. 20, 1966.
Pentagonal roof type reflecting mirror assemblies which can obviate the above disadvantage have been proposed by copending U.S. Pat. No. 4,206,991 issued on June 10, 1980, entitled "PENTAGONAL ROOF TYPE REFLECTING MIRROR ASSEMBLIES FOR SINGLE LENS REFLEX CAMERAS", assigned to the same assignee.
The following prior art has been cited in the above referenced pending applications: German Pat. No. 1,127,202, French Pat. No. 1,174,231, German Pat. No. 2,448,625, East German Pat. No. 50,419, Great Britain Pat. No. 1,051,433, U.S. Pat. No. 3,218,946, U.S. Pat. No. 3,620,147, U.S. Pat. No. 3,868,711, U.S. Pat. No. 4,063,261, U.S. Pat. No. 3,760,700, U.S. Pat. No. 3,182,576, U.S. Pat. No. 3,952,321, U.S. Pat. No. 3,911,457, U.S. Pat. No. 3,962,710, U.S. Pat. No. 3,387,530, U.S. Pat. No. 3,840,298, and U.S. Pat. No. 4,128,321.
However, another disadvantage has been found in the mirror assemblies disclosed in the copending applications in that vapor within the mirror assemblies can be condensed into dew on the mirror surfaces to thereby dim the image field of the viewfinder when the environmental atmosphere is cooled because the inside hollow space of the mirror assemblies is prevented from communicating with the exterior of the mirror assemblies.